This application relates to voltage indicators and, in particular, voltage indicators for indicating high voltages, such as AC transmission line voltages in the kilovolt range. The application relates in particular to probes for such voltage indicators.
A voltage indicator, in its simplest form, indicates the presence of a voltage near an energized power line or equipment by means of an annunciator, such as a buzzer, light or the like. More sophisticated voltage indicators may be calibrated or adjusted to indicate the presence of voltage only above a predetermined threshold and to give no indication of voltages below that threshold. Still more sophisticated indicators may give an actual indication of the magnitude of the voltage detected, by use of an analog meter display or a digital display.
Voltage indicators are, in general, different from voltmeters in that they are single-point measurement devices. In other words, they contact only one line or only one point and do not measure voltage with respect to a fixed reference, such as ground or other line of different phase. Voltage indicators are also generally not able to measure DC voltages, since DC measurements typically require a direct connection to both the energized conductor and a reference conductor, such as ground.
Without a direct connection to a reference conductor, a voltage indicator must determine the voltage on an energized conductor based on the strength of the electric field surrounding the conductor which, for a given geometry, is directly proportional to the voltage on the conductor. Thus, voltage indicators can give an estimation of the actual applied voltage by measuring the strength of the electric field surrounding the conductor. However, for a conductor at a given voltage, the strength of the electric field can very significantly, depending upon the geometry of the conductor, the distance between the energized conductor and other conductors or ground, and the placement of the voltage indicator on the conductor. For this reason, voltage indicators that display actual voltage have very large published measurement tolerances, usually +/−25%. The actual measurement error can approach +/−50% for measurements made near ground or a grounded electrical conductor.
The reason for this may be explained by reference to FIGS. 1–3. FIG. 1 shows an overhead high voltage electric line 10 supported on poles 11. The line generates an electric field 15 that surrounds the conductor. Electric fields can be pictured as a series of concentric circles 16 surrounding the conductor, called equipotential lines, because the electric potential or voltage is equal everywhere along the given line. Each of these field lines 16 represents a percentage of the actual line voltage. The high voltage line 10 itself is at 100% voltage, the innermost field line may represent 90% voltage, the next may represent 80%, and so on, out to the outermost circle, which may represent 10% voltage. The spacing between the equipotential lines indicates the strength of the electric field, with closer spacing corresponding to higher field strength.
The voltage indicator 20 in FIG. 1 includes a housing 21 which houses the electronic circuitry of the indicator and is mounted at the end of an elongated hot stick 23. The housing 21 carries a probe hook 25, which is connected to the circuitry in the housing 21. The voltage indicator 20 measures the strength of the electric field by measuring the voltage difference between the conductor 10 that the hook probe 25 is touching and the electric field a distance way from the conductor 10. In practice, the voltage indicator 20 is actually measuring a voltage between two electrodes, one being the hook probe 25 and the second being a conductive coating on the inside of the housing 21 to shield the electronic circuitry from the strong electric fields. Thus, this shielding coating serves as a reference electrode for measuring the strength of the electric field. It is known from calculation and testing that, for the geometry of a typical single overhead high-voltage conductor, the voltage indicator housing 21 will be located at about the 80% equipotential line when the hook probe 25 is placed over the conductor 10. Thus, the voltage indicator 20 will measure the difference between the conductor at 100% and the reference electrode at 80%, the difference between the two being 20% of the voltage. The voltage indicator circuitry is calibrated to measure and display 20% of voltage as the actual line voltage, in kilovolts.
This arrangement works reasonably well for overhead line conductors as they are typically arranged in power distributions systems. However, in a typical power distribution system, a high voltage line may travel some distance overhead on poles and then be connected to an underground cable to provide power to a residential or commercial subdivision. Commercially available voltage indicators typically have accessory probes available that are specialized for underground applications. The disadvantage of using voltage indicators at or near ground level is that they are generally calibrated for electric fields typical of overhead line geometry, and the geometry of equipment at or near ground levels is very different from overhead. FIG. 2 shows the electric field 15 from an overhead line 10, which is connected to underground equipment 17, and also shows the electric field 15A surrounding a terminal 18 on the equipment 17 mounted near ground at the same voltage. Electric fields are always distributed completely between an energized conductor and a grounded conductor or ground. If the ground is far way, as it typically is for overhead conductors, the electric field extends radially away from the conductor uniformly in all directions and spreads out over a great distance. If a grounded conductor or ground is close to an energized line or terminal, then the electric field is compressed into the shorter distance between the energized line or terminal and ground and the field strength is higher, because electric field strength is expressed as volts per distance. As can be seen in FIG. 3, the energized terminal 18 on the equipment 17 mounted on the ground is very close to both the grounded metal services of the equipment enclosure and the ground itself. When the voltage indicator probe hook 25 is placed on this terminal 18, the housing 21 is now disposed at about the 20% equipotential line. Thus, the voltage indicator 20 will measure the distance between the conductor at 100% and the reference electrode at 20%, the difference between the two being 80% of the voltage. Because the voltage indicator circuitry has been calibrated to measure and display only 20% of the voltage as the actual line voltage, it will now indicate a voltage magnitude that is substantially higher than the actual voltage on the equipment terminal 17.
Most commercially available voltage indicators have an optional accessory probe for voltage measurements on equipment at or near ground level. These probes are usually called “underground” probes, not necessarily because they are used underground, but because they are used on equipment that is connected to underground power cables. These probes typically provide only for making an electrical connection between the voltage indicator and the different types of terminals. These probes do not address the issue of inaccuracy of measurements made on this equipment.